The Theoretical Study Of Several Issues On Zeolitc Imidazolate Frameworks

Metal organic frameworks(MOFs)have coordination network structures which are composed of inorganic clusters and organic ligands.They have attracted a great deal of interest from many researchers due to their unique structural characteristics,rich chemical composition,and wide potential applications.Among them,zeolite imidazolate frameworks(ZIFs)can form glass via melting-quenching method,which further expands their application fields and attracts extensive attention.In order to calrify the glass formation mechanism of these ZIF materials and explore new ways to modulate the glass formation and properties,in this thesis we construct several continuous random network models of ZIFs to systematically study the electronic structures,atomic bonding and physical properties of different ZIF systems via first principles calculation;we also explore the modulation effects of different halogenated organic ligands on the glass formation of ZIF system.In addition,based on the molecular dynamics simulation,a new theoretical method has been developed,which can effectively calculate the mechanical properties of the material.This lays a theoretical foundation for the study of the elastic properties in super-large systems.The main innovations and conclusions of this thesis are as follows:1.We innovatively construct an amorphous ZIF(a-ZIF)model with continuous random network topology which is the same as SiO₂ glass and suitable for first-principles calculations.By using the density functional theory method,the electronic structure of the a-ZIF model and the bonding characteristics of atoms are systematically studied.It is confirmed that the relatively strong Zn-N bond plays a key role in maintaining the tetrahedral bonded network structure.The calculated optical properties of a-ZIF show a complex absorption spectrum with an ultra-low refractive index of1.327 and a plasmon frequency of 15.810 e V.2.Based on density functional theory(DFT)calculations,the effects of mixed organic ligands on the fundamental properties of amorphous ZIF-62,Zn(Im)_2-x(bIm)_xare systematically investigated.The results show that as the concentration of b Im increases,the cohesion of a-ZIF-62 also increases.In particular,due to the addition of b Im ligands,an obvious intermediate mid-band appears in the conduction band which further leads to alternation in the dielectric function.The calculated refractive index increases with the increase of b Im,which is in good agreement with the experimental results.3.We theoretically design two novel halogenated analogs of a-ZIFs by adding the halide functional groups X(X=—Cl,—Br)to the imidazolate linkers in the a-ZIFs.Density functional theory is used to investigate the effects of the halogen atoms on the structural,electronic and optical properties of the a-ZIFs.Our results show that the halogenated analogs of a-ZIF have slightly weaker internal cohesion and larger refractive index.A detailed analysis of electronic structure clarifies that the halogen atoms effectively tune the band gap,as well as the valance band and conduction band of a-ZIFs.Moreover,the calculated ultra-low dielectric constants and large energy loss functions indicate that they are potential candidates for electromagnetic absorptive materials.4.First-principles calculations are used to study the elastic properties and electronic structures of three zeolite imidazole frameworks(ZIF):ZIF-4,ZIF-62 and TIF-4,which have the same CaGa₂O₄ topology.Tensor analysis of the elastic constants reveals that these three materials have abnormal and highly anisotropic elastic behavior,and all have low Young's modulus and shear modulus.In addition,their flexibility incorporates regions of negative Poisson's ratio(NPR).The elastic properties of these ZIF crystals are affected by the substituted organic ligands.In the crystal structures,the bonding between Zn-N mainly exhibits ionic interaction,the others show covalent interaction in which the C-H bond has a certain polarization.5.An important amendment to the application of the stress-fluctuation method,and a general energy-strain scheme is proposed to calculate the elastic constants of materials under finite temperature and pressure conditions.This scheme can simplify the process of solving the elastic constants of materials with the stress-fluctuation method,and is valid for both liquid and solid state.In addition,this prediction is further verified by using a pairwise potential and a three-body otential,the Lennard-Jones(LJ)potential and the crystalline silicon with a Stillinger-Weber potential.